diff --git a/src/Avalonia.Base/Data/BindingValue.cs b/src/Avalonia.Base/Data/BindingValue.cs
index 93948e54ee..e8c84ffb21 100644
--- a/src/Avalonia.Base/Data/BindingValue.cs
+++ b/src/Avalonia.Base/Data/BindingValue.cs
@@ -247,7 +247,7 @@ namespace Avalonia.Data
                 UnsetValueType _ => Unset,
                 DoNothingType _ => DoNothing,
                 BindingNotification n => n.ToBindingValue().Cast<T>(),
-                _ => new BindingValue<T>((T)value)
+                _ => new BindingValue<T>((T?)value)
             };
         }
 
diff --git a/src/Avalonia.Base/Data/Converters/MethodToCommandConverter.cs b/src/Avalonia.Base/Data/Converters/MethodToCommandConverter.cs
index 4a4644317d..251dbb458d 100644
--- a/src/Avalonia.Base/Data/Converters/MethodToCommandConverter.cs
+++ b/src/Avalonia.Base/Data/Converters/MethodToCommandConverter.cs
@@ -161,7 +161,7 @@ namespace Avalonia.Data.Converters
                 .Compile();
         }
 
-        private static Expression? ConvertTarget(object? target, MethodInfo method) =>
+        private static Expression ConvertTarget(object target, MethodInfo method) =>
             target is null ? null : Expression.Convert(Expression.Constant(target), method.DeclaringType);
 
         internal class WeakPropertyChangedProxy
diff --git a/src/Avalonia.Controls/Application.cs b/src/Avalonia.Controls/Application.cs
index 157bebe02b..df55d9e250 100644
--- a/src/Avalonia.Controls/Application.cs
+++ b/src/Avalonia.Controls/Application.cs
@@ -104,7 +104,7 @@ namespace Avalonia
         /// <value>
         /// The application's focus manager.
         /// </value>
-        public IFocusManager FocusManager
+        public IFocusManager? FocusManager
         {
             get;
             private set;
@@ -116,7 +116,7 @@ namespace Avalonia
         /// <value>
         /// The application's input manager.
         /// </value>
-        public InputManager InputManager
+        public InputManager? InputManager
         {
             get;
             private set;
@@ -175,7 +175,7 @@ namespace Avalonia
         /// - <see cref="ISingleViewApplicationLifetime"/>
         /// - <see cref="IControlledApplicationLifetime"/> 
         /// </summary>
-        public IApplicationLifetime ApplicationLifetime { get; set; }
+        public IApplicationLifetime? ApplicationLifetime { get; set; }
 
         event Action<IReadOnlyList<IStyle>> IGlobalStyles.GlobalStylesAdded
         {
diff --git a/src/Avalonia.Controls/Flyouts/FlyoutBase.cs b/src/Avalonia.Controls/Flyouts/FlyoutBase.cs
index 4b903d056c..b874d149bf 100644
--- a/src/Avalonia.Controls/Flyouts/FlyoutBase.cs
+++ b/src/Avalonia.Controls/Flyouts/FlyoutBase.cs
@@ -562,8 +562,12 @@ namespace Avalonia.Controls.Primitives
             return eventArgs.Cancel;
         }
 
-        internal static void SetPresenterClasses(IControl presenter, Classes classes)
+        internal static void SetPresenterClasses(IControl? presenter, Classes classes)
         {
+            if(presenter is null)
+            {
+                return;
+            }
             //Remove any classes no longer in use, ignoring pseudo classes
             for (int i = presenter.Classes.Count - 1; i >= 0; i--)
             {
diff --git a/src/Avalonia.Controls/Primitives/Popup.cs b/src/Avalonia.Controls/Primitives/Popup.cs
index 4c9b95c22d..0f15f647dc 100644
--- a/src/Avalonia.Controls/Primitives/Popup.cs
+++ b/src/Avalonia.Controls/Primitives/Popup.cs
@@ -93,8 +93,8 @@ namespace Avalonia.Controls.Primitives
         public static readonly StyledProperty<bool> OverlayDismissEventPassThroughProperty =
             AvaloniaProperty.Register<Popup, bool>(nameof(OverlayDismissEventPassThrough));
 
-        public static readonly DirectProperty<Popup, IInputElement> OverlayInputPassThroughElementProperty =
-            AvaloniaProperty.RegisterDirect<Popup, IInputElement>(
+        public static readonly DirectProperty<Popup, IInputElement?> OverlayInputPassThroughElementProperty =
+            AvaloniaProperty.RegisterDirect<Popup, IInputElement?>(
                 nameof(OverlayInputPassThroughElement),
                 o => o.OverlayInputPassThroughElement,
                 (o, v) => o.OverlayInputPassThroughElement = v);
@@ -138,7 +138,7 @@ namespace Avalonia.Controls.Primitives
         private bool _isOpen;
         private bool _ignoreIsOpenChanged;
         private PopupOpenState? _openState;
-        private IInputElement _overlayInputPassThroughElement;
+        private IInputElement? _overlayInputPassThroughElement;
         private Action<IPopupHost?>? _popupHostChangedHandler;
 
         /// <summary>
@@ -310,7 +310,7 @@ namespace Avalonia.Controls.Primitives
         /// Gets or sets an element that should receive pointer input events even when underneath
         /// the popup's overlay.
         /// </summary>
-        public IInputElement OverlayInputPassThroughElement
+        public IInputElement? OverlayInputPassThroughElement
         {
             get => _overlayInputPassThroughElement;
             set => SetAndRaise(OverlayInputPassThroughElementProperty, ref _overlayInputPassThroughElement, value);
diff --git a/src/Avalonia.Controls/SplitView.cs b/src/Avalonia.Controls/SplitView.cs
index 0e35c610b2..6a0d4e2023 100644
--- a/src/Avalonia.Controls/SplitView.cs
+++ b/src/Avalonia.Controls/SplitView.cs
@@ -129,14 +129,14 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="Pane"/> property
         /// </summary>
-        public static readonly StyledProperty<object?> PaneProperty =
-            AvaloniaProperty.Register<SplitView, object?>(nameof(Pane));
+        public static readonly StyledProperty<object> PaneProperty =
+            AvaloniaProperty.Register<SplitView, object>(nameof(Pane));
 
         /// <summary>
         /// Defines the <see cref="PaneTemplate"/> property.
         /// </summary>
-        public static readonly StyledProperty<IDataTemplate?> PaneTemplateProperty =
-            AvaloniaProperty.Register<HeaderedContentControl, IDataTemplate?>(nameof(PaneTemplate));
+        public static readonly StyledProperty<IDataTemplate> PaneTemplateProperty =
+            AvaloniaProperty.Register<HeaderedContentControl, IDataTemplate>(nameof(PaneTemplate));
 
         /// <summary>
         /// Defines the <see cref="UseLightDismissOverlayMode"/> property
@@ -267,7 +267,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets or sets the data template used to display the header content of the control.
         /// </summary>
-        public IDataTemplate? PaneTemplate
+        public IDataTemplate PaneTemplate
         {
             get => GetValue(PaneTemplateProperty);
             set => SetValue(PaneTemplateProperty, value);
diff --git a/src/Avalonia.Controls/TrayIcon.cs b/src/Avalonia.Controls/TrayIcon.cs
index 59edb6278a..9fbc2fc8b2 100644
--- a/src/Avalonia.Controls/TrayIcon.cs
+++ b/src/Avalonia.Controls/TrayIcon.cs
@@ -140,7 +140,7 @@ namespace Avalonia.Controls
         /// Gets or sets the parameter to pass to the <see cref="Command"/> property of a
         /// <see cref="TrayIcon"/>.
         /// </summary>
-        public object CommandParameter
+        public object? CommandParameter
         {
             get { return GetValue(CommandParameterProperty); }
             set { SetValue(CommandParameterProperty, value); }
diff --git a/src/Avalonia.Input/ICommandSource.cs b/src/Avalonia.Input/ICommandSource.cs
index eed71759d5..410b3a2e47 100644
--- a/src/Avalonia.Input/ICommandSource.cs
+++ b/src/Avalonia.Input/ICommandSource.cs
@@ -1,5 +1,5 @@
 using System.Windows.Input;
-
+#nullable enable
 namespace Avalonia.Input
 {
     ///<summary>
@@ -12,13 +12,13 @@ namespace Avalonia.Input
         /// Classes that implement this interface should enable or disable based on the command's CanExecute return value.
         /// The property may be implemented as read-write if desired.
         /// </summary>
-        ICommand Command { get; }
+        ICommand? Command { get; }
 
         /// <summary>
         /// The parameter that will be passed to the command when executing the command.
         /// The property may be implemented as read-write if desired.
         /// </summary>
-        object CommandParameter { get; }
+        object? CommandParameter { get; }
 
 
         /// <summary>
diff --git a/src/Avalonia.Styling/Styling/PropertySetterInstance.cs b/src/Avalonia.Styling/Styling/PropertySetterInstance.cs
index 1c3055fed6..bb0e342df6 100644
--- a/src/Avalonia.Styling/Styling/PropertySetterInstance.cs
+++ b/src/Avalonia.Styling/Styling/PropertySetterInstance.cs
@@ -16,14 +16,14 @@ namespace Avalonia.Styling
         private readonly IStyleable _target;
         private readonly StyledPropertyBase<T>? _styledProperty;
         private readonly DirectPropertyBase<T>? _directProperty;
-        private readonly T _value;
+        private readonly T? _value;
         private IDisposable? _subscription;
         private bool _isActive;
 
         public PropertySetterInstance(
             IStyleable target,
             StyledPropertyBase<T> property,
-            T value)
+            T? value)
         {
             _target = target;
             _styledProperty = property;
@@ -57,7 +57,7 @@ namespace Avalonia.Styling
             {
                 if (_styledProperty is object)
                 {
-                    _subscription = _target.SetValue(_styledProperty, _value, BindingPriority.Style);
+                    _subscription = _target.SetValue(_styledProperty!, _value, BindingPriority.Style);
                 }
                 else
                 {
diff --git a/src/Avalonia.Styling/Styling/Setter.cs b/src/Avalonia.Styling/Styling/Setter.cs
index 1f3d6335a9..4c94ea02dd 100644
--- a/src/Avalonia.Styling/Styling/Setter.cs
+++ b/src/Avalonia.Styling/Styling/Setter.cs
@@ -101,7 +101,7 @@ namespace Avalonia.Styling
                 data.result = new PropertySetterInstance<T>(
                     data.target,
                     property,
-                    (T)data.value);
+                    (T?)data.value);
             }
         }
 
@@ -128,7 +128,7 @@ namespace Avalonia.Styling
                 data.result = new PropertySetterInstance<T>(
                     data.target,
                     property,
-                    (T)data.value);
+                    (T)data.value!);
             }
         }
 
diff --git a/src/tools/MicroComGenerator/CSharpGen.Utils.cs b/src/tools/MicroComGenerator/CSharpGen.Utils.cs
index da845b0ecd..28baaa65f8 100644
--- a/src/tools/MicroComGenerator/CSharpGen.Utils.cs
+++ b/src/tools/MicroComGenerator/CSharpGen.Utils.cs
@@ -40,7 +40,7 @@ namespace MicroComGenerator
         SyntaxToken Semicolon() => Token(SyntaxKind.SemicolonToken);
 
         static VariableDeclarationSyntax DeclareVar(string type, string name,
-            ExpressionSyntax? initializer = null)
+            ExpressionSyntax initializer = null)
             => VariableDeclaration(ParseTypeName(type),
                 SingletonSeparatedList(VariableDeclarator(name)
                     .WithInitializer(initializer == null ? null : EqualsValueClause(initializer))));